Internet Engineering Task Force (IETF) R. Despres
Request for Comments: 7600 RD-IPtech
Category: Experimental S. Jiang, Ed.
ISSN: 2070-1721 Huawei Technologies Co., Ltd
R. Penno
Cisco Systems, Inc.
Y. Lee
Comcast
G. Chen
China Mobile
M. Chen
BBIX, Inc.
July 2015
IPv4 Residual Deployment via IPv6 - A Stateless Solution (4rd)
Abstract
This document specifies a stateless solution for service providers to
progressively deploy IPv6-only network domains while still offering
IPv4 service to customers. The solution's distinctive properties are
that TCP/UDP IPv4 packets are valid TCP/UDP IPv6 packets during
domain traversal and that IPv4 fragmentation rules are fully
preserved end to end. Each customer can be assigned one public IPv4
address, several public IPv4 addresses, or a shared address with a
restricted port set.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7600.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Despres, et al. Experimental [Page 2]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 20151. Introduction
For service providers to progressively deploy IPv6-only network
domains while still offering IPv4 service to customers, the need for
a stateless solution, i.e., one where no per-customer state is needed
in IPv4-IPv6 gateway nodes of the provider, has been discussed in
[Solutions-4v6]. This document specifies one such solution, named
"4rd" for IPv4 Residual Deployment. Its distinctive properties are
that TCP/UDP IPv4 packets are valid TCP/UDP IPv6 packets during
domain traversal and that IPv4 fragmentation rules are fully
preserved end to end.
Using this solution, IPv4 packets are transparently tunneled across
IPv6 networks (the reverse of IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) [RFC5969], in which IPv6 packets are
statelessly tunneled across IPv4 networks).
While IPv6 headers are too long to be mapped into IPv4 headers (which
is why 6rd requires encapsulation of full IPv6 packets in IPv4
packets), IPv4 headers can be reversibly translated into IPv6 headers
in such a way that, during IPv6 domain traversal, UDP packets having
checksums and TCP packets are valid IPv6 packets. IPv6-only
middleboxes that perform deep packet inspection can operate on them,
in particular for port inspection and web caches.
In order to deal with the IPv4 address shortage, customers can be
assigned shared public IPv4 addresses with statically assigned
restricted port sets. As such, it is a particular application of the
Address plus Port (A+P) approach [RFC6346].
Deploying 4rd in networks that have enough public IPv4 addresses,
customer sites can also be assigned full public IPv4 addresses. 4rd
also supports scenarios where a set of public IPv4 addresses are
assigned to customer sites.
The design of 4rd builds on a number of previous proposals made for
IPv4-via-IPv6 transition technologies (Section 7).
In some use cases, IPv4-only applications of 4rd-capable customer
nodes can also work with stateful NAT64s [RFC6146], provided these
are upgraded to support 4rd tunnels in addition to their IP/ICMP
translation [RFC6145]. The advantage is then a more complete IPv4
transparency than with double translation.
How the 4rd model fits in the Internet architecture is summarized in
Section 3. The protocol specifications are detailed in Section 4.
Sections 5 and 6 deal with security considerations and IANA
considerations, respectively. Previous proposals that influenced
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
this specification are listed in Section 7. A few typical 4rd use
cases are presented in Appendices A, B, C, D, and E.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
ISP: Internet Service Provider. In this document, the service it
offers can be DSL, fiber-optics, cable, or mobile. The ISP can
also be a private-network operator.
4rd (IPv4 Residual Deployment): An extension of the IPv4 service
where public IPv4 addresses can be statically shared among
several customer sites, each one being assigned an exclusive
port set. This service is supported across IPv6-routing
domains.
4rd domain (or Domain): An ISP-operated IPv6 network across which
4rd is supported according to the present specification.
Tunnel packet: An IPv6 packet that transparently conveys an IPv4
packet across a 4rd domain. Its header has enough information
to reconstitute the IPv4 header at Domain exit. Its payload is
the original IPv4 payload.
CE (Customer Edge): A customer-side tunnel endpoint. It can be in a
node that is a host, a router, or both.
BR (Border Relay): An ISP-side tunnel endpoint. Because its
operation is stateless (neither per CE nor per session state),
it can be replicated in as many nodes as needed for scalability.
4rd IPv6 address: IPv6 address used as the destination of a Tunnel
packet sent to a CE or a BR.
NAT64+: An ISP NAT64 [RFC6146] that is upgraded to support 4rd
tunneling when IPv6 addresses it deals with are 4rd IPv6
addresses.
4rd IPv4 address: A public IPv4 address or, in the case of a shared
public IPv4 address, a public transport address (public IPv4
address plus port number).
PSID (Port-Set Identifier): A flexible-length field that
algorithmically identifies a port set.
Despres, et al. Experimental [Page 5]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
4rd IPv4 prefix: A flexible-length prefix that may be a public IPv4
prefix, a public IPv4 address, or a public IPv4 address followed
by a PSID.
Mapping rule: A set of parameters that are used by BRs and CEs to
derive 4rd IPv6 addresses from 4rd IPv4 addresses. Mapping
rules are also used by each CE to derive a 4rd IPv4 prefix from
an IPv6 prefix that has been delegated to it.
EA bits (Embedded Address bits): Bits that are the same in a 4rd
IPv4 address and in the 4rd IPv6 address derived from it.
BR Mapping rule: The Mapping rule that is applicable to off-domain
IPv4 addresses (addresses reachable via BRs). It can also apply
to some or all CE-assigned IPv4 addresses.
CE Mapping rule: A Mapping rule that is applicable only to
CE-assigned IPv4 addresses (shared or not).
NAT64+ Mapping rule: The Mapping rule that is applicable to IPv4
addresses reachable via a NAT64+.
CNP (Checksum Neutrality Preserver): A field of 4rd IPv6 addresses
that ensures that TCP-like checksums do not change when IPv4
addresses are replaced with 4rd IPv6 addresses.
4rd Tag: A 16-bit tag whose value allows 4rd CEs, BRs, and NAT64+s
to distinguish 4rd IPv6 addresses from other IPv6 addresses.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 20153. The 4rd Model
4rd Domain
+-----------------------------+
| IPv6 routing |
| Enforced ingress filtering | +----------
... | | |
| +------+
Customer site | |BR(s) | IPv4
+------------+ | BR IPv6 prefix --> |and/or| Internet
| dual-stack | | |N4T64+|
| +--+ | +------+
| |CE+-+ <-- a CE IPv6 prefix | |
| +--+ | | +----------
| | | |
+------------+ | <--IPv4 tunnels--> +------------
=> Derived | (Mesh or hub-and-spoke |
4rd IPv4 prefix| topologies) | IPv6
| | Internet
... | |
| +------------
+-----------------------------+
<== one or several Mapping rules
(e.g., announced to CEs in stateless DHCPv6)
Figure 1: The 4rd Model in the Internet Architecture
How the 4rd model fits in the Internet architecture is represented in
Figure 1.
A 4rd domain is an IPv6 network that includes one or several 4rd BRs
or NAT64+s at its border with the public IPv4 Internet and that can
advertise its IPv4-IPv6 Mapping rule(s) to CEs according to
Section 4.9.
BRs of a 4rd Domain are all identical as far as 4rd is concerned. In
a 4rd CE, the IPv4 packets that need to reach a BR will be
transformed (as detailed in Section 4.3) into IPv6 packets that have
the same anycast IPv6 prefix, which is the 80-bit BR prefix, in their
destination addresses. They are then routed to any of the BRs. The
80-bit BR IPv6 prefix is an arbitrarily chosen /64 prefix from the
IPv6 address space of the network operator and appended with 0x0300
(16-bit 4rd Tag; see R-9 in Section 4.5).
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
Using the Mapping rule that applies, each CE derives its 4rd IPv4
prefix from its delegated IPv6 prefix, or one of them if it has
several; see Section 4.4 for details. If the obtained IPv4 prefix
has more than 32 bits, the assigned IPv4 address is shared among
several CEs. Bits beyond the first 32 specify a set of ports whose
use is reserved for the CE.
IPv4 traffic is automatically tunneled across the Domain, in either
mesh topology or hub-and-spoke topology [RFC4925]. By default, IPv4
traffic between two CEs follows a direct IPv6 route between them
(mesh topology). If the ISP configures the hub-and-spoke option,
each IPv4 packet from one CE to another is routed via a BR.
During Domain traversal, each tunneled TCP/UDP IPv4 packet looks like
a valid TCP/UDP IPv6 packet. Thus, TCP/UDP access control lists that
apply to IPv6, and possibly some other functions using deep packet
inspection, also apply to IPv4.
In order for IPv4 anti-spoofing protection in CEs and BRs to remain
effective when combined with 4rd tunneling, ingress filtering
[RFC3704] has to be in effect in IPv6 (see R-12 and Section 5).
If an ISP wishes to support dynamic IPv4 address sharing in addition
to or in place of 4rd stateless address sharing, it can do so by
means of a stateful NAT64. By upgrading this NAT to add support for
4rd tunnels, which makes it a NAT64+, CEs that are assigned no static
IPv4 space can benefit from complete IPv4 transparency between the CE
and the NAT64. (Without this NAT64 upgrade, IPv4 traffic is
translated to IPv6 and back to IPv4, during which time the DF =
MF = 1 combination for IPv4, as recommended for host fragmentation in
Section 8 of [RFC4821], is lost.)
IPv4 packets are kept unchanged by Domain traversal, except that:
o The IPv4 Time To Live (TTL), unless it is 1 or 255 at Domain
entry, decreases during Domain traversal by the number of
traversed routers. This is acceptable because it is undetectable
end to end and also because TTL values that can be used with some
protocols to test the adjacency of communicating routers are
preserved [RFC4271] [RFC5082]. The effect on the traceroute
utility, which uses TTL expiry to discover routers of end-to-end
paths, is noted in Section 4.3.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
o IPv4 packets whose lengths are <= 68 octets always have their
"Don't Fragment" (DF) flags set to 1 at Domain exit even if they
had DF = 0 at Domain entry. This is acceptable because these
packets are too short to be fragmented [RFC791] and so their DF
bits have no meaning. Besides, both [RFC1191] and [RFC4821]
recommend that sources always set DF to 1.
o Unless the Tunnel Traffic Class option applies to a Domain
(Section 4.2), IPv4 packets may have their Type of Service (TOS)
fields modified after Domain traversal (Section 4.7).
4. Protocol Specifications
This section describes detailed 4rd protocol specifications. They
are mainly organized by functions. As a brief summary:
o A 4rd CE MUST follow R-1, R-2, R-3, R-4, R-6, R-7, R-8, R-9, R-10,
R-11, R-12, R-13, R-14, R-16, R-17, R-18, R-19, R-20, R-21, R-22,
R-23, R-24, R-25, R-26, and R-27.
o A 4rd BR MUST follow R-2, R-3, R-4, R-5, R-6, R-9, R-12, R-13,
R-14, R-15, R-19, R-20, R-21, R-22, and R-24.
4.1. NAT44 on CE
R-1: A CE node that is assigned a shared public IPv4 address MUST
include a NAT44 [RFC3022]. This NAT44 MUST only use external
ports that are in the CE-assigned port set.
NOTE: This specification only concerns IPv4 communication between
IPv4-capable endpoints. For communication between IPv4-only
endpoints and IPv6-only remote endpoints, the "Bump-in-the-Host"
(BIH) specification [RFC6535] can be used. It can coexist in a node
with the CE function, including scenarios where the IPv4-only
function is a NAT44 [RFC3022].
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 20154.2. Mapping Rules and Other Domain Parameters
R-2: CEs and BRs MUST be configured with the following Domain
parameters:
A. One or several Mapping rules, each one comprising the
following:
1. Rule IPv4 prefix
2. EA-bits length
3. Rule IPv6 prefix
4. Well-Known Ports (WKPs) authorized (OPTIONAL)
B. Domain Path MTU (PMTU)
C. Hub-and-spoke topology (Yes or No)
D. Tunnel Traffic Class (OPTIONAL)
"Rule IPv4 prefix" is used to find, by a longest match, which Mapping
rule applies to a 4rd IPv4 address (Section 4.5). A Mapping rule
whose Rule IPv4 prefix is longer than /0 is a CE Mapping rule. BR
and NAT64+ Mapping rules, which must apply to all off-domain IPv4
addresses, have /0 as their Rule IPv4 prefixes.
"EA-bits length" is the number of bits that are common to 4rd IPv4
addresses and 4rd IPv6 addresses derived from them. In a CE Mapping
rule, it is also the number of bits that are common to a CE-delegated
IPv6 prefix and the 4rd IPv4 prefix derived from it. BR and NAT64+
Mapping rules have EA-bits lengths equal to 32.
"Rule IPv6 prefix" is the prefix that is used as a substitute for the
Rule IPv4 prefix when a 4rd IPv6 address is derived from a 4rd IPv4
address (Section 4.5). In a BR Mapping rule or a NAT64+ Mapping
rule, it MUST be a /80 prefix whose bits 64-79 are the 4rd Tag.
"WKPs authorized" may be set for Mapping rules that assign shared
IPv4 addresses to CEs. (These rules are those whose length of the
Rule IPv4 prefix plus the EA-bits length exceeds 32.) If set,
well-known ports may be assigned to some CEs having particular IPv6
prefixes. If not set, fairness is privileged: all IPv6 prefixes
concerned with the Mapping rule have port sets having identical
values (no port set includes any of the well-known ports).
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
"Domain PMTU" is the IPv6 Path MTU that the ISP can guarantee for all
of its IPv6 paths between CEs and between BRs and CEs. It MUST be at
least 1280 octets [RFC2460].
"Hub-and-spoke topology", if set to Yes, requires CEs to tunnel all
IPv4 packets via BRs. If set to No, CE-to-CE packets take the same
routes as native IPv6 packets between the same CEs (mesh topology).
"Tunnel Traffic Class", if provided, is the IPv6 traffic class that
BRs and CEs MUST set in Tunnel packets. In this case, evolutions of
the IPv6 traffic class that may occur during Domain traversal are not
reflected in TOS fields of IPv4 packets at Domain exit (Section 4.7).
4.3. Reversible Packet Translations at Domain Entries and Exits
R-3: Domain-entry nodes that receive IPv4 packets with IPv4 options
MUST discard these packets and return ICMPv4 error messages to
signal IPv4-option incompatibility (Type = 12, Code = 0,
Pointer = 20) [RFC792]. This limitation is acceptable because
there are a lot of firewalls in the current IPv4 Internet that
also filter IPv4 packets with IPv4 options.
R-4: Domain-entry nodes that receive IPv4 packets without IPv4
options MUST convert them to Tunnel packets, with or without
IPv6 fragment headers, depending on what is needed to ensure
IPv4 transparency (Figure 2). Domain-exit nodes MUST convert
them back to IPv4 packets.
An IPv6 fragmentation header MUST be included at tunnel entry
(Figure 2) if and only if one or several of the following
conditions hold:
* The Tunnel Traffic Class option applies to the Domain.
* TTL = 1 OR TTL = 255.
* The IPv4 packet is already fragmented, or may be fragmented
later on, i.e., if MF = 1 OR offset > 0 OR (total length >
68 AND DF = 0).
In order to optimize cases where fragmentation headers are
unnecessary, the NAT44 of a CE that has one SHOULD send packets
with TTL = 254.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
NOTE 2: TTL treatment has been chosen to permit adjacency tests
between two IPv4 nodes situated at both ends of a 4rd tunnel. TTL
values to be preserved for this are TTL = 255 and TTL = 1. For other
values, TTL decreases between two IPv4 nodes as though the traversed
IPv6 routers were IPv4 routers.
The effect of this TTL treatment on IPv4 traceroute is specific:
(1) the number of routers of the end-to-end path includes traversed
IPv6 routers; (2) IPv6 routers of a Domain are listed after IPv4
routers of Domain entry and exit; (3) the IPv4 address shown for an
IPv6 router is the IPv6-only dummy IPv4 address (Section 4.8);
(4) the response time indicated for an IPv6 router is that of the
next router.
NOTE 3: Provided the sum of obtained IPv4 addresses and protocol
matches Addr_Prot_Cksm. If not, the packet MUST be silently
discarded.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
it has more than 32 bits, the CE MUST take the first 32 bits as
its shared public IPv4 address and bits beyond the first 32 as
its Port-Set identifier (PSID). Ports of its restricted port
set are by default those that have any non-zero value in their
first 4 bits (the PSID offset), followed by the PSID, and
followed by any values in remaining bits. If the WKP
authorized option applies to the Mapping rule, there is no
4-bit offset before the PSID so that all ports can be assigned.
NOTE: The choice of the default PSID position in port fields
has been guided by the following objectives: (1) for fairness,
avoid having any of the well-known ports 0-1023 in the port set
specified by any PSID value; (2) for compatibility with RTP/
RTCP [RFC4961], include in each port set pairs of consecutive
ports; (3) in order to facilitate operation and training, have
the PSID at a fixed position in port fields; (4) in order to
facilitate documentation in hexadecimal notation, and to
facilitate maintenance, have this position nibble-aligned.
Ports that are excluded from assignment to CEs are 0-4095,
instead of just 0-1023, in a trade-off to favor nibble
alignment of PSIDs and overall simplicity.
R-8: A CE whose delegated IPv6 prefix has its longest match with the
Rule IPv6 prefix of the BR Mapping rule MUST take as its IPv4
address the 32 bits that, in the delegated IPv6 prefix, follow
this Rule IPv6 prefix. If this is the case while the hub-and-
spoke option applies to the Domain, or if the Rule IPv6 prefix
is not a /80, there is a configuration error in the Domain. An
implementation-dependent administrative action MAY be taken.
A CE whose delegated IPv6 prefix does not match the Rule IPv6
prefix of either any CE Mapping rule or the BR Mapping rule,
and is in a Domain that has a NAT64+ Mapping rule, MUST be
noted as having the unspecified IPv4 address.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
R-9: BRs, and CEs that are assigned public IPv4 addresses, shared or
not, MUST derive 4rd IPv6 addresses from 4rd IPv4 addresses via
the steps below or their functional equivalent (Figure 5
details the shared public IPv4 address case):
NOTE: The rules for forming 4rd-specific Interface Identifiers
(IIDs) are to obey [RFC7136]:
"Specifications of other forms of 64-bit IIDs MUST specify how
all 64 bits are set."
and
"the whole IID value MUST be viewed as an opaque bit string by
third parties, except possibly in the local context."
(1) If hub-and-spoke topology does not apply to the Domain, or
if it applies but the IPv6 address to be derived is a
source address from a CE or a destination address from a
BR, find the CE Mapping rule whose Rule IPv4 prefix has
the longest match with the IPv4 address.
If no Mapping rule is thus obtained, take the BR Mapping
rule.
If the obtained Mapping rule assigns IPv4 prefixes to CEs,
i.e., if the length of the Rule IPv4 prefix plus EA-bits
length is 32 - k, with k >= 0, delete the last k bits of
the IPv4 address.
Otherwise, if the length of the Rule IPv4 prefix plus the
EA-bits length is 32 + k, with k > 0, take k as the PSID
length and append to the IPv4 address the PSID copied from
bits p to p+3 of the Port_or_ICMP_ID field where (1) p,
the PSID offset, is 4 by default and 0 if the WKPs
authorized option applies to the rule; (2) the
Port_or_ICMP_ID field is in bits of the IP payload that
depend on whether the address is source or destination, on
whether the packet is ICMP or not, and, if it is ICMP,
whether it is an error message or an Echo message. This
field is:
a. If the packet Protocol is not ICMP, the port field
associated with the address (bits 0-15 for a source
address and bits 16-31 for a destination address).
b. If the packet is an ICMPv4 Echo or Echo reply message,
the ICMPv4 Identification field (bits 32-47).
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
c. If the packet is an ICMPv4 error message, the port
field associated with the address in the returned
packet header (bits 240-255 for a source address and
bits 224-239 for a destination address).
NOTE 1: Using Identification fields of ICMP messages as
port fields permits the exchange of Echo requests and Echo
replies between shared-address CEs and IPv4 hosts having
exclusive IPv4 addresses. Echo exchanges between two
shared-address CEs remain impossible, but this is a
limitation inherent in address sharing (one reason among
many to use IPv6).
NOTE 2: When the PSID is taken in the port fields of the
IPv4 payload, implementation is kept independent from any
particular Layer 4 protocol having such port fields by not
checking that the protocol is indeed one that has such
port fields. A packet may consequently go, in the case of
a source mistake, from a BR to a shared-address CE with a
protocol that is not supported by this CE. In this case,
the CE NAT44 returns an ICMPv4 "protocol unreachable"
error message. The IPv4 source is thus appropriately
informed of its mistake.
(2) In the result, replace the Rule IPv4 prefix with the Rule
IPv6 prefix.
(3) If the result is shorter than a /64, append to the result
a null padding up to 64 bits, followed by the 4rd Tag
(0x0300), and followed by the IPv4 address.
NOTE: The 4rd Tag is a 4rd-specific mark. Its function is
to ensure that 4rd IPv6 addresses are recognizable by CEs
without any interference with the choice of subnet
prefixes in CE sites. (These choices may have been done
before 4rd is enabled.)
For this, the 4rd Tag has its "u" and "g" bits [RFC4291]
both set to 1, so that they maximally differ from these
existing IPv6 address schemas. So far, u = g = 1 has not
been used in any IPv6 addressing architecture.
With the 4rd Tag, IPv6 packets can be routed to the 4rd
function within a CE node based on a /80 prefix that no
native IPv6 address can contain.
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
(4) Add to the result a Checksum Neutrality Preserver (CNP).
Its value, in one's complement arithmetic, is the opposite
of the sum of 16-bit fields of the IPv6 address other than
the IPv4 address and the CNP themselves (i.e., five
consecutive fields in address bits 0-79).
NOTE: The CNP guarantees that Tunnel packets are valid
IPv6 packets for all Layer 4 protocols that use the same
checksum algorithm as TCP. This guarantee does not depend
on where the checksum fields of these protocols are placed
in IP payloads. (Today, such protocols are UDP [RFC768],
TCP [RFC793], UDP-Lite [RFC3828], and the Datagram
Congestion Control Protocol (DCCP) [RFC5595]. Should new
ones be specified, BRs will support them without needing
an update.)
R-10: A 4rd-capable CE SHOULD, and a 4rd-enabled CE MUST, always
prohibit all addresses that use its advertised prefix and have
an IID starting with 0x0300 (4rd Tag), by using Duplicate
Address Detection [RFC4862].
R-11: A CE that is assigned the unspecified IPv4 address (see
Section 4.4) MUST use, for packets tunneled between itself and
the Domain NAT64+, addresses as detailed in Figure 6: part (a)
for its IPv6 source, and part (b) as IPv6 destinations that
depend on IPv4 destinations. A NAT64+, being NAT64 conforming
[RFC6146], MUST accept IPv6 packets whose destination conforms
to Figure 6(b) (4rd Tag instead of "u" and 0x00 octets). In
its Binding Information Base, it MUST remember whether a
mapping was created with a "u" or 4rd-tag destination. In the
IPv4-to-IPv6 direction, it MUST use 4rd tunneling, with source
address conforming to Figure 6(b), when using a mapping that
was created with a 4rd-tag destination.
+---------------------+---------+-------+-------------+------+
(a) | CE IPv6 prefix | 0 |4rd Tag| 0 | CNP |
+---------------------+---------+-------+-------------+------+
: <= 64 : >= 0 : 16 : 32 : 16 :
4rd IPv6 address of a CE having no public IPv4 address
<----------- Rule IPv6 prefix --------->:
+-------------------------------+-------+-------------+------+
(b) | NAT64+ IPv6 prefix |4rd Tag|IPv4 address | CNP |
+-------------------------------+-------+-------------+------+
: 64 : 16 : 32 : 16 :
4rd IPv6 address of a host reachable via a NAT64+
Figure 6: Rules for CE and NAT64+
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RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
R-12: For anti-spoofing protection, CEs and BRs MUST check that the
IPv6 source address of each received Tunnel packet is that
which, according to R-9, is derived from the source 4rd IPv4
address. For this, the IPv4 address used to obtain the source
4rd IPv4 address is that embedded in the IPv6 source address
(in its bits 80-111). (This verification is needed because
IPv6 ingress filtering [RFC3704] applies only to IPv6 prefixes,
without any guarantee that Tunnel packets are built as
specified in R-9.)
R-13: For additional protection against packet corruption at a link
layer that might be undetected at this layer during Domain
traversal, CEs and BRs SHOULD verify that source and
destination IPv6 addresses have not been modified. This can be
done by checking that they remain checksum neutral (see the
Note above regarding the CNP).
4.6. Fragmentation Processing4.6.1. Fragmentation at Domain Entry
R-14: If an IPv4 packet enters a CE or BR with a size such that the
derived Tunnel packet would be longer than the Domain PMTU, the
packet has to be either discarded or fragmented. The
Domain-entry node MUST discard it if the packet has DF = 1,
with an ICMP error message returned to the source. It MUST
fragment it otherwise, with the payload of each fragment not
exceeding PMTU - 48. The first fragment has its offset equal
to the received offset. Subsequent fragments have offsets
increased by the lengths of the payloads of previous fragments.
Functionally, fragmentation is supposed to be done in IPv4
before applying reversible header translation to each fragment;
see Section 4.3.
Despres, et al. Experimental [Page 23]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 20154.6.2. Ports of Fragments Addressed to Shared-Address CEs
Because ports are available only in the first fragments of IPv4
fragmented packets, a BR needs a mechanism to send to the right
shared-address CEs all fragments of fragmented packets.
For this, a BR MAY systematically reassemble fragmented IPv4 packets
before tunneling them. But this consumes large memory space, creates
opportunities for denial-of-service-attacks, and can significantly
increase forwarding delays. This is the reason for the following
requirement:
R-15: BRs SHOULD support an algorithm whereby received IPv4 packets
can be forwarded on the fly. The following is an example of
such an algorithm:
(1) At BR initialization, if at least one CE Mapping rule
deals with one or more shared public IPv4 addresses (i.e.,
length of Rule IPv4 prefix + EA-bits length > 32), the BR
initializes an empty "IPv4 packet table" whose entries
have the following items:
- IPv4 source
- IPv4 destination
- IPv4 identification
- Destination port
(2) When the BR receives an IPv4 packet whose matching Mapping
rule deals with one or more shared public IPv4 addresses
(i.e., length of Rule IPv4 prefix + EA-bits length > 32),
the BR searches the table for an entry whose IPv4 source,
IPv4 destination, and IPv4 identification are those of the
received packet. The BR then performs actions as detailed
in Table 5, depending on which conditions hold.
Despres, et al. Experimental [Page 24]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 20154.6.3. Packet Identifications from Shared-Address CEs
When packets go from CEs that share the same IPv4 address to a common
destination, a precaution is needed to guarantee that packet
identifications set by sources are different. Otherwise, packet
reassembly at the destination could be confused because it is based
only on source IPv4 address and Identification. The probability of
such confusing situations may in theory be very low, but a safe
solution is needed in order to avoid creating new attack
opportunities.
R-18: A CE that is assigned a shared public IPv4 address MUST only
use packet identifications that have the CE PSID in their
bits 0 to PSID length - 1.
R-19: A BR or a CE that receives a packet from a shared-address CE
MUST check that bits 0 to PSID length - 1 of their packet
identifications are equal to the PSID found in the source 4rd
IPv4 address.
4.7. TOS and Traffic Class Processing
IPv4 TOS and IPv6 traffic class have the same semantic, that of the
differentiated services field, or DS field, specified in [RFC2474]
and [RFC6040]. Their first 6 bits contain a differentiated services
codepoint (DSCP), and their last 2 bits can convey explicit
congestion notifications (ECNs), which both may evolve during Domain
traversal. [RFC2983] discusses how the DSCP can be handled by tunnel
endpoints. The Tunnel Traffic Class option provides permission to
ignore DS-field evolutions occurring during Domain traversal, if the
desired behavior is that of generic tunnels conforming to [RFC2473].
R-20: Unless the Tunnel Traffic Class option is configured for the
Domain, BRs and CEs MUST copy the IPv4 TOS into the IPv6
traffic class at Domain entry and copy back the IPv6 traffic
class into the IPv4 TOS at Domain exit.
R-21: If the Tunnel Traffic Class option is configured for a Domain,
BRs and CEs MUST at Domain entry take the configured Tunnel
Traffic Class as the IPv6 traffic class and copy the received
IPv4 TOS into the IPv4_TOS of the fragment header (Figure 3).
At Domain exit, they MUST copy back the IPv4_TOS of the
fragment header into the IPv4 TOS.
Despres, et al. Experimental [Page 26]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 20154.8. Tunnel-Generated ICMPv6 Error Messages
If a Tunnel packet is discarded on its way across a 4rd domain
because of an unreachable destination, an ICMPv6 error message is
returned to the IPv6 source. For the IPv4 source of the discarded
packet to be informed of packet loss, the ICMPv6 message has to be
converted into an ICMPv4 message.
R-22: If a CE or BR receives an ICMPv6 error message [RFC4443], it
MUST synthesize an ICMPv4 error packet [RFC792]. This packet
MUST contain the first 8 octets of the discarded packet's IP
payload. The reserved IPv4 dummy address (192.0.0.8/32; see
Section 6) MUST be used as its source address.
As described in [RFC6145], ICMPv6 Type = 1 and Code = 0
(Destination Unreachable, No route to destination) MUST be
translated into ICMPv4 Type = 3 and Code = 0 (Destination
Unreachable, Net unreachable), and ICMPv6 Type = 3 and Code = 0
(Time Exceeded, Hop limit exceeded in transit) MUST be
translated into ICMPv4 Type = 11 and Code = 0 (Time Exceeded,
time to live exceeded in transit).
4.9. Provisioning 4rd Parameters to CEs
Domain parameters listed in Section 4.2 are subject to the following
constraints:
R-23: Each Domain MUST have a BR Mapping rule and/or a NAT64+ Mapping
rule. The BR Mapping rule is only used by CEs that are
assigned public IPv4 addresses, shared or not. The NAT64+
Mapping rule is only used by CEs that are assigned the
unspecified IPv4 address (Section 4.4) and therefore need an
ISP NAT64 to reach IPv4 destinations.
R-24: Each CE and each BR MUST support up to 32 Mapping rules.
Support for up to 32 Mapping rules ensures that independently
acquired CEs and BR nodes can always interwork.
ISPs that need Mapping rules for more IPv4 prefixes than this
number SHOULD split their networks into multiple Domains.
Communication between these domains can be done in IPv4 or by
some other implementation-dependent, but equivalent, means.
Despres, et al. Experimental [Page 27]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
Fragmentation-related attacks
As discussed in Section 4.6, each BR of a Domain that assigns
shared public IPv4 addresses should maintain a dynamic table of
fragmented packets that go to these shared-address CEs.
This leaves BRs vulnerable to denial-of-service attacks from hosts
that would send very large numbers of first fragments and would
never send last fragments having the same packet identifications.
This vulnerability is inherent in IPv4 address sharing, be it
static or dynamic. Compared to what it is with algorithms that
reassemble IPv4 packets in BRs, it is, however, significantly
mitigated by the algorithm provided in Section 4.6.2, as that
algorithm uses much less memory space.
6. IANA Considerations
IANA has allocated the following:
o Encapsulated options OPTION_4RD (97), OPTION_4RD_MAP_RULE (98),
and OPTION_4RD_NON_MAP_RULE (99). These options have been
recorded in the option code space of the "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)" registry. See
Section 4.9 of this document and Section 24.3 of [RFC3315]).
Value | Description | Reference
-----------+-------------------------+---------------
97 | OPTION_4RD | this document
98 | OPTION_4RD_MAP_RULE | this document
99 | OPTION_4RD_NON_MAP_RULE | this document
o Reserved IPv4 address 192.0.0.8/32 to be used as the "IPv4 dummy
address" (Section 4.8).
7. Relationship with Previous Works
The present specification has been influenced by many previous IETF
drafts, in particular those accessible at
<http://tools.ietf.org/html/draft-xxxx>, where "xxxx" refers to the
following (listed in order, by date of their first versions):
o bagnulo-behave-nat64 (2008-06-10)
o xli-behave-ivi (2008-07-06)
o despres-sam-scenarios (2008-09-28)
o boucadair-port-range (2008-10-23)
Despres, et al. Experimental [Page 31]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015Appendix A. Textual Representation of Mapping Rules
In the sections that follow, each Mapping rule will be represented as
follows, using 0bXXX to represent binary number XXX; square brackets
("[ ]") indicate optional items:
{Rule IPv4 prefix, EA-bits length, Rule IPv6 prefix
[, WKPs authorized]}
EXAMPLES:
{0.0.0.0/0, 32, 2001:db8:0:1:300::/80}
a BR Mapping rule
{198.16.0.0/14, 22, 2001:db8:4000::/34}
a CE Mapping rule
{0.0.0.0/0, 32, 2001:db8:0:1::/80}
a NAT64+ Mapping rule
{198.16.0.0/14, 22, 2001:db8:4000::/34, Yes}
a CE Mapping rule
and hub-and-spoke topology
Appendix B. Configuring Multiple Mapping Rules
As far as Mapping rules are concerned, the simplest deployment model
is that in which the Domain has only one rule (the BR Mapping rule).
To assign an IPv4 address to a CE in this model, an IPv6 /112 is
assigned to it, comprising the BR /64 prefix, the 4rd Tag, and the
IPv4 address. However, this model has the following limitations: (1)
shared IPv4 addresses are not supported; (2) IPv6 prefixes used for
4rd are too long to also be used for native IPv6 addresses; (3) if
the IPv4 address space of the ISP is split with many disjoint IPv4
prefixes, the IPv6 routing plan must be as complex as an IPv4 routing
plan based on these prefixes.
With more Mapping rules, CE prefixes used for 4rd can be those used
for native IPv6. How to choose CE Mapping rules for a particular
deployment does not need to be standardized.
The following is only a particular pragmatic approach that can be
used for various deployment scenarios. It is applied in some of the
use cases that follow.
(1) Select a "Common_IPv6_prefix" that will appear at the beginning
of all 4rd CE IPv6 prefixes.
(2) Choose all IPv4 prefixes to be used, and assign one of them to
each CE Mapping rule i.
Despres, et al. Experimental [Page 37]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
F. Take Rule IPv6 prefix(i) = the Common_IPv6_prefix followed
by Rule_code(i).
:<--------------------- L(CE IPv6 prefix(i)) --------------------->:
: :
: 32 - L(Rule IPv4 prefix(i)) PSID_length(i):
: \ | :
: :<---------'--------><--'-->:
: : || :
: : \/ :
: :<------->:<--- EA-bits length(i) --->:
: L(Rule_code(i))
: : :
+----------------------------+---------+
| Common_IPv6_prefix |Rule_code|
| | (i) |
+----------------------------+---------+
:<------ L(Rule IPv6 prefix(i)) ------>:
Figure 10: Diagram of One Pragmatic Approach
Appendix C. Adding Shared IPv4 Addresses to an IPv6 NetworkC.1. With CEs within CPEs
Here, we consider an ISP that offers IPv6-only service to up to 2^20
customers. Each customer is delegated a /56, starting with common
prefix 2001:db8:0::/36. The ISP wants to add public IPv4 service for
customers that are 4rd capable. It prefers to do so with stateless
operation in its nodes but has significantly fewer IPv4 addresses
than IPv6 addresses, so a sharing ratio is necessary.
The only IPv4 prefixes it can use are 192.8.0.0/15, 192.4.0.0/16,
192.2.0.0/16, and 192.1.0.0/16 (neither overlapping nor
aggregatable). This gives 2^(32 - 15) + 3 * 2^(32 - 16) IPv4
addresses, i.e., 2^18 + 2^16. For the 2^20 customers to have the
same sharing ratio, the number of IPv4 addresses to be shared has to
be a power of 2. The ISP can therefore give up using one of its
/16s, say the last one. (Whether or not it could be motivated to
return it to its Internet Registry is off-scope for this document.)
The sharing ratio to apply is then 2^20 / 2^18 = 2^2 = 4, giving a
PSID length of 2.
Despres, et al. Experimental [Page 39]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
An IPv4 packet sent to address 192.4.238.238 and port 7777 is
tunneled to the IPv6 address obtained as follows (Section 4.5):
IPv4 address : 192.4.238.238 (0xc004 eeee)
: 0b1100 0000 0000 0100 1110 1110 1110 1110
Rule IPv4 prefix(i): 192.4.0.0/16 (longest match)
: 0b1100 0000 0000 0100
IPv4 suffix(i) : 0b1110 1110 1110 1110
EA-bits length(i) : 18
PSID length(i) : 2 (= 16 + 18 - 32)
Port field : 0b 0001 1110 0110 0001 (7777)
PSID : 0b11
Rule IPv6 prefix(i): 2001:0db8:0800::/38
CE IPv6 prefix : 2001:0db8:0bbb:bb00::/56
IPv6 address : 2001:0db8:0bbb:bb00:300:c004:eeee:YYYY
with YYYY = the computed CNP
C.2. With Some CEs behind Third-Party Router CPEs
We now consider an ISP that has the same need as the ISP described in
the previous section, except that (1) instead of using only its own
IPv6 infrastructure, it uses that of a third-party provider and (2)
some of its customers use this provider's Customer Premises Equipment
(CPEs) so that they can use specific services offered by the
provider. In these CPEs, a non-zero index is used to route IPv6
packets to the physical port to which CEs are attached, say 0x2.
Each such CPE delegates to the CE nodes the customer-site IPv6 prefix
followed by this index.
The ISP is supposed to have the same IPv4 prefixes as those in the
previous use case -- 192.8.0.0/15, 192.4.0.0/16, and 192.2.0.0/16 --
and to use the same Common_IPv6_prefix, 2001:db8:0::/36.
We also assume that only a minority of customers use third-party
CPEs, so that it is sufficient to use only one of the two /16s for
them.
Mapping rules are then (see Appendix C.1):
{192.8.0.0/15, 19, 2001:0db8:0000::/37}
{192.4.0.0/16, 18, 2001:0db8:0800::/38}
{192.2.0.0/16, 18, 2001:0db8:0c00::/38}
{0.0.0.0/0, 32, 2001:0db8:0000:0001:300::/80}
CEs that are behind third-party CPEs derive their own IPv4 addresses
and port sets as described in Appendix C.1.
Despres, et al. Experimental [Page 41]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
In a BR, and also in a CE if the topology is mesh, the IPv6 address
that is derived from IPv4 address 192.4.238.238 and port 7777 is
obtained as described in the previous section, except for the last
two steps, which are modified as follows:
IPv4 address : 192.4.238.238 (0xc004 eeee)
: 0b1100 0000 0000 0100 1110 1110 1110 1110
Rule IPv4 prefix(i): 192.4.0.0/16 (longest match)
: 0b1100 0000 0000 0100
IPv4 suffix(i) : 0b1110 1110 1110 1110
EA-bits length(i) : 18
PSID length(i) : 2 (= 16 + 18 - 32)
Port field : 0b 0001 1110 0110 0001 (7777)
PSID : 0b11
Rule IPv6 prefix(i): 2001:0db8:0800::/38
CE IPv6 prefix : 2001:0db8:0bbb:bb00::/60
IPv6 address : 2001:0db8:0bbb:bb00:300:192.4.238.238:YYYY
with YYYY = the computed CNP
Appendix D. Replacing Dual-Stack Routing with IPv6-Only Routing
In this use case, we consider an ISP that offers IPv4 service with
public addresses individually assigned to its customers. It also
offers IPv6 service, as it has deployed dual-stack routing. Because
it provides its own CPEs to customers, it can upgrade all of its CPEs
to support 4rd. It wishes to take advantage of this capability to
replace dual-stack routing with IPv6-only routing, without changing
any IPv4 address or IPv6 prefix.
For this, the ISP can use the single-rule model described at the
beginning of Appendix B. If the prefix routed to BRs is chosen to
start with 2001:db8:0:1::/64, this rule is:
{0.0.0.0/0, 32, 2001:db8:0:1:300::/80}
All that is needed in the network before disabling IPv4 routing is
the following:
o In all routers, where there is an IPv4 route toward x.x.x.x/n, add
a parallel route toward 2001:db8:0:1:300:x.x.x.x::/(80+n).
o Where IPv4 address x.x.x.x was assigned to a CPE, now delegate
IPv6 prefix 2001:db8:0:1:300:x.x.x.x::/112.
Despres, et al. Experimental [Page 42]

RFC 7600 Stateless IPv4 Residual Deployment (4rd) July 2015
NOTE: In parallel with this deployment, or after it, shared IPv4
addresses can be assigned to IPv6 customers. It is sufficient that
IPv4 prefixes used for this be different from those used for
exclusive-address assignments. Under this constraint, Mapping rules
can be set up according to the same principles as those described in
Appendix C.
Appendix E. Adding IPv6 and 4rd Service to a Net-10 Network
In this use case, we consider an ISP that has only deployed IPv4,
possibly because some of its network devices are not yet IPv6
capable. Because it did not have enough IPv4 addresses, it has
assigned private IPv4 addresses [RFC1918] to customers, say 10.x.x.x.
It thus supports up to 2^24 customers (a "Net-10" network, using the
NAT444 model [NAT444]).
Now, it wishes to offer IPv6 service without further delay, using 6rd
[RFC5969]. It also wishes to offer incoming IPv4 connectivity to its
customers with a simpler solution than that provided by the Port
Control Protocol (PCP) [RFC6887].
This appendix describes an example that adds IPv6 (using 6rd) and 4rd
services to the "Net-10" private IPv4 network.
The IPv6 prefix to be used for 6rd is supposed to be 2001:db8::/32,
and the public IPv4 prefix to be used for shared addresses is
supposed to be 198.16.0.0/16 (0xc610). The resulting sharing ratio
is 2^24 / 2^(32 - 16) = 256, giving a PSID length of 8.
The ISP installs one or several BRs at its border to the public IPv4
Internet. They support 6rd, and 4rd above it. The BR prefix /64 is
supposed to be that which is derived from IPv4 address 10.0.0.1
(i.e., 2001:db8:0:100:/64).
In accordance with [RFC5969], 6rd BRs are configured with the
following parameters: IPv4MaskLen = 8; 6rdPrefix = 2001:db8::/32;
6rdBRIPv4Address = 192.168.0.1 (0xc0a80001).
4rd Mapping rules are then the following:
{198.16.0.0/16, 24, 2001:db8:0:0:300::/80}
{0.0.0.0/0, 32, 2001:db8:0:100:300:/80,}
Any customer device that supports 4rd in addition to 6rd can then use
its assigned shared IPv4 address with 240 assigned ports.
Despres, et al. Experimental [Page 43]